Groundwater under or adjacent a structure's foundation can cause serious damage. In addition to increased concrete deterioration and accelerated rebar corrosion of structure such as walls and pilings, internal dampness and resultant high humidity accelerates corrosion of expensive electrical and mechanical equipment, such as what may be placed within a dam or in a crawl space beneath a building and can increase maintenance requirements through frequent repainting or cleaning to combat mold and mildew growth. Furthermore, the intruding water raises the relative humidity thereby accelerating the corrosion rate of mechanical equipment in the area and creating unacceptable air quality, for example, in any air drawn from above a crawl space into a building. This air pollution carries concomitant health problems due to the rapid growth of bacteria and mold.
In selective problem areas, the usual approach to the treatment of water intrusion problems is to “trench and drain.” In other words, to excavate and expose a piling area or the base of a foundation and to install a drain tile system around the manmade structure or affected area. Other areas, such as floors, are untreatable using conventional methods.
Electro-osmosis has origins in 1809, when F. F. Reuss originally described an experiment demonstrating that water could be forced to flow through a clay-water system when an external electric field was applied to it. Research since then has shown that water surrounding cations moves with them, the flow initiated by the predominant movement of cations present in the pore fluid of porous media such as a clay, concrete, concrete block, brick, cementitious construction materials, or the like. The basic physics and chemistry of electro-osmosis can be found in several textbooks and treatises. Glasstone, S., Textbook of Physical Chemistry, 2d ed., D. Van Nostrand Company, Inc., Princeton, N.J., 1946. Tikhomolova, K. P., Electro-Osmosis, Ellis Horwood Limited, Chichester, West Sussex, England, 1993.
Electro-osmosis is typically used to solve the problem of groundwater intrusion, which can cause serious damage to a structure's foundation and interior. Electro-Osmotic Pulse (EOP) technology typically offers an alternative that addresses moisture problems from the interior of affected areas without the cost of excavation. Examples of such systems are described below.
In one system, humidity is removed from a damp structure by positioning electrodes within the structure and applying a DC voltage across them. U.S. Pat. No. 3,856,646, Methods and Electrodes for the Drying of Damp Buildings, to Morarau, Dec. 24, 1974.
In another system, chloride ions are removed from concrete by embedding an anode in an electrolyte and establishing an electric current between the anode and the concrete structure in order to avoid corrosion of the concrete's reinforcing means, typically steel rebar. U.S. Pat. No. 5,296,120, Apparatus for the Removal of Chloride from Reinforced Concrete Structures, to Bennett et al., Mar. 22, 1994.
Another system discloses a process for changing the bond strength between concrete and its steel reinforcement by passing DC current through the concrete. U.S. Pat. No. 5,312,526, Method for Increasing or Decreasing Bond Strength Between Concrete and Embedded Steel, and for Sealing the Concrete-to-Steel Interface, to Miller, May 17, 1994.
Still another method used to eliminate humidity from concrete uses electro-osmosis to pass current pulses in a predetermined pattern through the concrete. U.S. Pat. No. 5,368,709, Method and Apparatus for Controlling the Relative Humidity in Concrete and Masonry Structures, to Utklev, Nov. 29, 1994.
A method that claims improvement over existing methods by choice of a narrow range of relationships among the three pulse durations of the pulse train provides longer anode life while optimizing the process of dehydration. U.S. Pat. No. 5,755,945, Method for Dehydrating Capillary Materials, to Kristiansen, May 26, 1998.
An improvement over previous methods claims to increase anode life while optimizing dehydration and the time to effect it. It uses a specific pulse train in which the positive pulse width is much greater than the negative pulse width that is, in turn, greater than the off period. U.S. Pat. No. 6,117,295, Method for Dehydrating a Porous Material, to Bjerke, Sep. 12, 2000.
A method that claims to be an improvement over the '709 patent provides a control unit to control the pulse width of individual pulses by monitoring characteristics of the energizing source. U.S. Pat. No. 6,126,802, Method and Device for Regulating and Optimizing Transport of Humidity by Means of Electroosmosis, to Utklev, Oct. 3, 2000.
A more recent patent proposes a solution to overcome the disadvantage of the '709 patent when used to dehumidify steel-reinforced structures. It specifically prevents the deterioration of the reinforcing steel by providing a second voltage to the reinforcement steel in addition to the typical electro-osmosis configuration of the '709 patent and its predecessors. U.S. Pat. No. 6,370,643 B1, Method for Effecting Fluid Flow in Porous Materials, to Finnebraaten, Aug. 7, 2001.
In a conventional method, an Electro-Osmotic Pulse (EOP) system is configured by installing anodes (positive electrodes) in the interior wall, floor or ceiling of the structure and cathodes (negative electrodes) in the soil exterior to the structure. Due to the extreme electrochemical environment surrounding the anode, special material and geometry requirements may be placed on an anode intended to be used for other than “trickle current” loads or for extended periods, or both.
Durable, dimensionally stable anodes are a recent development in anode technology. U.S. Pat. No. 5,055,169, to Hock et al., Method of Making Mixed Metal Oxide Coated Substrates, Oct. 8, 1991, describes a reactive ion plating process using RF, specifically identifying the rate of evaporation of a noble metal such as ruthenium or iridium, the rate of evaporation of a valve metal such as titanium, and the partial oxygen pressure needed to produce an electrically conductive mixed metal oxide ceramic coating on a valve metal substrate. The coated substrate can sustain a current density of 150 A per m2 of exposed coating surface in fresh water electrolyte, preferably for at least 20 hours, and more preferably for at least 75 hours, without an excessive increase in the voltage level required to maintain that current density. As noted in the '169 patent abstract, these anodes have excellent characteristics to include: low resistivity, very low dissolution rates, long life, excellent durability, and excellent corrosion resistance. Durable, dimensionally stable anodes are also referred to as semiconductive anodes. Durable anodes that are classified as dimensionally stable generally consist of a valve metal substrate such as niobium, tantalum, titanium or alloys thereof, with a catalytic coating consisting of precious metal(s), most often from the platinum metal group, and often in oxide form in combination with valve metal oxides as a mixed metal oxide.